Add support for rebar deps that don't specify hash

[rgrinberg]

Here's an example of such a dependency in the wild: (rebar indeed supports it) https://github.com/processone/cache_tab/blob/master/rebar.config#L3

[5HT]

That's cool. Thanks! :-)

[rgrinberg]

Np. So is the patch accepted?

[5HT]

Yep.